Development of Cost Effective and High Performance Composite for CO2 Capture in Ca-Cu Looping Process

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Peer-review under responsibility of the organizing committee of GHGT-13.

doi: 10.1016/j.egypro.2017.03.1163

Energy Procedia 114 ( 2017 ) 211 – 219

ScienceDirect

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland

Development of Cost Effective and High Performance Composite for CO

2

Capture in Ca-Cu Looping Process

Saima Sultana Kazi

^a

, Asunción Aranda

^a

, Luca di Felice

^a

, Julien Meyer

^a,

* Ramón Murillo

^b

, Gemma Grasa

^b

aInstitute for Energy Technology (IFE), Department of Environmental Technology, Instituttveien 18, 2007, Kjeller, Norway

bInstituto de Carboquímica (ICB-CSIC),C/Miguel Luesma Castán 4, 50018, Zaragoza, Spain

Abstract

New cost effective and high performance bi-functional composite materials consisting of CaO and CuO supported on Ca12Al14O33 have been developed for the calcium-copper (Ca-Cu) looping process. A hydrothermal method has been used to synthesize the materials from low cost precursors. Prepared composites are characterised by X-ray diffraction, scanning electron microscopy together with energy-dispersive spectroscopy mapping, and by crushing strength tests. Long term chemical performances of the materials are tested in a thermo-gravimetric analyser. The composite containing 53 wt% CuO, 22 wt% CaO and 25 wt% Ca12Al14O33 shows stable CO2 capture (~15 g-CO2/100g composite) and O2 carrying capacity (~7 g-O2/100g composite) in long term Ca-Cu multi-cycle, and shows satisfactory mechanical strength compared to reference materials used in the Ca-Cu process.

Keywords: Ca-Cu looping process; Hydrothermal synthesis; Composite materials; CO₂capture capacity; O₂ carrying capacity.

1. Introduction

Fossil fuel combustion is the major contributor for increasing CO2 concentration in the atmosphere which

* Corresponding author. Tel.: +47-99-46-08-95 E-mail address: [email protected]

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consequently leads to global warming. CO2 capture and storage technology (CCS) is claimed to have the potential to reduce these CO2 emissions by producing concentrated CO2 streams from fossil fuel power plants or industry intensive industries ready for transportation to safe storage sites. The three main approaches to capture CO2 are post- and pre-combustion capture, as well as oxy-fuel combustion. The most common pre-combustion process uses steam reforming with amine scrubbing as CO2 capture technology to produce H2 from natural gas [1]. An alternative approach for H2 production from hydrocarbons is the sorption enhanced reforming (SER) process with in-situ removal of CO2 by means of CaO-based high temperature sorbent. The key advantages of SER are: (i) Overall process simplification and intensification, (ii) lower reforming reaction temperature and (iii) higher hydrogen yield (95 to 99 mol %, dry basis) compared to conventional steam reforming [2, 3].

Recently, a cost effective and energy efficient calcium-copper looping (Ca-Cu) process has been proposed by Abanades et al. for hydrogen production with CO2 capture [4, 5]. The Ca-Cu process is a hybrid between the SER process (combining reforming and a CaO/CaCO3 cycle for high temperature CO2 capture) and the chemical looping process (where a metal oxide provides O2 and heat in the reduction reaction of the fuel). This coupling allows a gaseous fuel to be oxidized in a N2 free environment, avoiding the use of an expensive air separation unit (ASU) for oxy-fuel combustion of the fuel [6-8], and shows potential for lower capture cost [9-12]. In this integrated process, two endothermic-exothermic reaction pairs are matched in the same reactor. The energy needed to drive the endothermic reforming reaction is provided by the heat produced in-situ from the carbonation reaction of the CaO- based sorbent. Similarly, the energy required for the calcination of CaCO3 is matched with the one produced in-situ by the reduction of the metal oxide, i.e. CuO.

In the Ca-Cu process, the chemical and thermal performance of the materials over cycles is the key to achieve high energy and CO2 capture efficiencies at reasonable cost. The decrease in performance of Ca-Cu looping materials is due to the presence of CuO (melting point, 1358 K) which makes it vulnerable at high temperature. De Diego et al. [13] and Qin et al. [14] showed that the oxygen carrying capacity of pure CuO decreases rapidly with repeated oxidation/reduction cycles due to agglomeration of CuO. Therefore, CuO needs to be combined with an inert support to avoid agglomeration. The inert support (e.g. Al2O3, Ca12Al14O33) enhances (1) the reactivity of CaO and CuO, (2) the mechanical strength, and (3) attrition resistance [9, 13]. Separate particles of CaO and CuO can be employed in the Ca-Cu looping process [5] to minimize the effect of CuO agglomeration on the reversibility of the sorbent. On the other hand, composite particles of CaO and CuO have the advantage of transferring heat efficiently for the endothermic calcination of CaCO3. Moreover, composite materials can reduce the overall amount of inert support in the materials, potentially reducing their cost [15]. Several efforts have been made to develop a single composite for Ca-Cu looping to enhance the cyclic stability of CaO/CuO [4, 14, 16]. Anthony et al. [17] designed a core-in-shell CaO/CuO composite to provide heat for the regeneration of CaCO3. Qin et al. [18] studied the behaviour of CaO/CuO based composite in a combined Ca-Cu looping process. Kierzkowska et al. [10] developed Cu-functionalized CaO based sorbents using a co-precipitation method. All the results reported a decrease in CO2

capture and O2 carrying capacity in cyclic carbonation/calcination and oxidation/reduction cycles, respectively.

Therefore, more efforts are necessary to enhance the performance of CaO-CuO composites for use in the Ca-Cu looping process.

The objective of the present work was to develop a cost effective and high performance CaO-CuO composite material by optimizing the total inert fraction of the solids. For this purpose, a recently developed simple and cost effective synthesis method [19] to produce CaO/Ca12Al14O33 sorbent from low cost precursors, based on the decomposition of hydrogarnet (Ca3Al2(OH)12), has been adapted to produce CaO-CuO composites.

2. Experimental

2.1. Synthesis of bi-functional CaO-CuO composite materials

Bi-functional CaO-CuO based composite materials with different mayenite (Ca12Al14O33) inert contents (15 wt%, 20 wt% and 25 wt %) were synthesized by hydrothermal method using Ca(OH)2 ( Emsure, 96 %), AlOOH (Sasol, 95 %), and CuO (Alfa Aesar, 99.9 %) as the precursors. The CuO content in the prepared composites was 50-60 wt% and the molar ratio of CuO/CaO equal to 2. Stoichiometric amounts of the precursors wereweighed and mixed together in isopropanol using a magnetic stirrer to obtain homogeneous slurry. Isopropanol was evaporated at 383 K

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overnight. After evaporation of the isopropanol, the dried powders were transferred to a 600 cm³ stainless steel autoclave. Deionized water was added to the powder at a liquid to solid weight ratio equal to 3. The slurry was heated to 423 K for 5 h in closed autoclave for hydrothermal synthesis. After completion of the hydrothermal reaction, the product from the autoclave was cooled down and dried in a heating cabinet at 383 K. The dried powders were calcined in air at 1273 K for 1 h (heating rate, 10 K.min^-1) to obtain the final composite material.

2.2. Preparation of composite particles by pelletization

CaO-CuO composite powders produced from hydrothermal synthesis were pelletized using a uniaxial press (50 MPa). The pellets were calcined in air at 1273 K for 1 h. Finally, the calcined pellets were crushed and sieved to the desired particle size (0.8 mm < dp < 1 mm).

2.3. Preparation of composite particles by high shear agglomeration

Agglomerates of CaO-CuO composite particles were prepared using a high shear wet granulator (GMX-LAB Micro, Vector Corporation). Before agglomeration, hydrothermally synthesized CaO-CuO composite powders were carbonated at 923 K in 100 vol% CO2 in a tube furnace to avoid stickiness to the granulator bowl (prevent formation of Ca(OH)2).

About 70 g of the carbonated composite powders were loaded into the high shear granulator bowl. The impeller and chopper speeds were adjusted to 1500 and 3600 rpm, respectively. The composite powders were pre-mixed for 5 min. A 15 wt% polyethylene glycol (PEG 4000) binder solution was added through the peristaltic pump to the powder until the agglomerates formed. The measured torque value from the impeller was used to control the agglomeration process. After completion of the agglomeration process, the particles were dried at 383 K overnight.

Dried particles were then calcined in air at 1273 K for 1 h to remove the organic binder and to partially sinter the granulated composite material. Finally, the granulated calcined composites were sieved to the size fraction of 0.8 mm < dp < 1 mm.

2.4. Preparation of epoxy embedded composite particles

A mixture of resin (Epofix) and hardener (Epofix) was prepared with a ratio of 15 and 2 parts by volume. The mixture was stirred gently with a rod so that no air was entrapped into the solution. The prepared mixture was then poured over few composite particles kept in the mould and solidified at room temperature. The epoxy-embedded specimen was polished with abrasive papers of 500, 800, 1000 (silicon carbide, SiC) until a smooth surface of the particles appears. Finally, the surface was polished with a diamond paste.

2.5. Characterization of composites

The constituent phases of the CaO-CuO composite materials were identified by X-ray diffraction (XRD), using a Bruker D8 advanced diffractometer equipment with CuKα radiation (λ = 1.5406 Å, with 40 kV and 40 mA). The XRD diffractograms were obtained in the angular range of 10-90º using step-size 0.02º and the scan time per step was 2 seconds. The cross-section of selected composite particles was studied by Scanning Electron Microscopy (SEM), (Hitachi S-4800N), with Energy-Dispersive Spectroscopy (EDS) mapping. Prior to imaging, composite particles embedded in epoxy were sputter-coated with carbon to inhibit charging of the particles. To evaluate the mechanical strength of the composite particles, the crushing strength was measured using a manually operated digital force gauge (Shimpo, FGV-10X).

The material compositions and their names corresponding to the components and weight percentages are given in Table 1.

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Table 1. Compositions and names of the prepared CaO-CuO-Ca₁₂Al₁₄O₃₃composites.

Material composition Name

61 wt% CuO/ 24 wt% CaO/ 15 wt% Ca12Al14O33 Cu61- Ca24- CA15 57 wt% CuO/ 23 wt% CaO/ 20 wt% Ca12Al14O33 Cu57- Ca23- CA20 53 wt% CuO/ 22 wt% CaO/ 25 wt% Ca12Al14O33 Cu53- Ca22- CA25

2.6. Performance of the bi-functional composites over multiple Ca-Cu looping cycles

The cyclic CO2 capture and O2 carrying capacity of the CaO-CuO composite materials were evaluated using a thermo-gravimetric analyser (TGA, CI electronics).

A small amount (~ 40 mg, size fraction, 0.8 < dp < 1mm) of calcined material was placed in an alumina crucible and heated up to 1143 K at a rate of 9 K.min^-1 under 100 vol% N2. The following cycle describes the main steps of the Ca-Cu looping process, i.e. reforming/carbonation, oxidation, reduction/calcination. When the temperature reaches 1143 K, reduction was performed in an atmosphere containing 60 vol% CO2, 20 vol% H2 and 20 vol% H2O.

After a reduction time of 2 min, temperature was lowered to 923 K in 70 vol% N2, 25 vol% H2O and 5 vol% H2. Carbonation, simulating the reforming step in the Ca-Cu process, was performed at 923 K in 15 vol% CO2, 25 vol%

H2O and 60 vol% N2 for 25 min. After the carbonation, temperature was increased again to 1143 K in 90 vol% CO2

and 10 vol% H2O. Oxidation was then performed at 1143 K for 20 min in an atmosphere containing 75 vol% CO2

and 25 vol% air to re-oxidise Cu to CuO. In the next step, calcination of CaCO3 in presence of 40 vol% CO2, 25 vol% H2O, and 35 vol% N2 wasperformed. For all composites, carbonation/calcination and oxidation/reduction cycles were repeated over 40 cycles to determine the long term behaviour of the composites. The cyclic CO2 capture capacity (g-CO2/100g composite) and O2 carrying capacity (g-O2/100g composite) are calculated from the weight changes during carbonation/calcination and oxidation/reduction as a function of time. The multi-cycle steps and testing conditions are summarized in Table 2.

Table 2. Steps and testing conditions of Ca-Cu looping process in TGA (Total gas flow, Ftotal = 500 cm³/min).

Reduction Cooling Carbonation Heating Oxidation Calcination

1143K, 2min 60 vol% CO2

20 vol% H₂ 20vol% H₂O

1143K to 923K 70 vol% N2

25 vol% H₂O 5 vol% H₂

923K, 25 min 15 vol% CO2

25 vol% H₂O 60 vol% N₂

923K to 1143K 90 vol% CO2

10 vol% H₂O

1143K, 20min 75 vol% CO2

25 vol% air

1143K, 15min 40 vol% CO2

25 vol% H₂O 35 vol% N₂

3. Results and discussions

Fig.1 shows that XRD diffractograms of the composites after calcination at 1273 K for 1 h. All composites exhibit three different constituent phases, tenorite (CuO), calcium oxide (CaO) and mayenite (Ca12Al14O33). These phases are confirmed by their characteristic main peaks, CuO (2θ = 35.8, 39.1, 49.30, 61.78, 68.45), CaO (2θ = 32.51, 37.50, 54.21, 79.92) and Ca12Al14O33 (2θ = 18.23, 33.34, 41.33, 55.15 and 57.55), respectively.

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Fig. 1. X-ray diffraction patterns of the prepared composites after calcination in air at 1273 K for 1 h; Phases: (●) CaO; (♦) CuO; () Ca₁₂Al₁₄O₃₃.

The CO2 capture and the O2 carrying capacity of the three composite materials (pelletized and agglomerated) in long term carbonation/calcination and oxidation/reduction multi-cycles are plotted in Figs. 2, 3 and 4. Fig. 2a and 2b show the CO2 capture and O2 carrying capacity of the agglomerated composite Cu61-Ca24-CA15. The composite loses half of its CO2 capture capacity after 15 cycles and stabilizes at ~10 g-CO2/100g composite after 42 cycles.

The O2 carrying capacity of the material decreases linearly with the number of cycles and attains 4.2 g-O2/100g composite after 42 cycles (Fig. 2b).

Fig. 2. (a) CO₂ capture and (b) O₂ carrying capacity of CaO-CuO composite (Cu61-Ca24-CA15) over 42 cycles in TGA; Carbonation temperature is 923 K; Calcination/oxidation/reduction temperature is 1143 K.

The composite material Cu57-Ca23-CA20 (pelletized and agglomerated) shows a maximum CO2 capture capacity of 18 g-CO2/100g composite for the 1^st cycle, declining slowly to reach 15.5 g-CO2/100g composite after 44 cycles, but without reaching stabilization (Fig. 3a). The pelletized and agglomerated composites show approximately the same behaviour. On the other hand, Fig. 3b shows that the initial O2 carrying capacity (~10 g- O2/100g composite) of the composite (pelletized and agglomerated) declines rapidly and stabilizes at ~5.3 g- O2/100g composite after 30 cycles.

( ( (

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Fig. 3. (a) CO2 capture and (b) O2 carrying capacity of CaO-CuO composite (Cu57-Ca23-CA20) over 44 cycles in TGA; Carbonation temperature is 923 K; Calcination/oxidation/reduction temperature is 1143 K.

Stabilisation of both CO2 capture and O2 carrying capacity is observed for the Cu53-Ca22-CA25 composite (pelletized and agglomerated). The initial CO2 capture capacity of ~16 g-CO2/100g composite stabilizes to ~15 g- CO2/100g composite after 50 cycles for both the pelletized and agglomerated composites (Fig. 4a). Fig. 4b shows that the O2 carrying capacities of both materials decrease from ~ 9.7 g-O2/100g composite to reach a stable value of

~ 7.3 g-O2/100g composite after 50 cycles.

Fig. 4. (a) CO₂ capture and (b) O₂ carrying capacity of CaO-CuO based composite (Cu53-Ca22-CA25) over 50 cycles in TGA; Carbonation temperature is 923 K; Calcination/oxidation/reduction temperature is 1143 K.

The results from TGA multi-cycle tests for the prepared composite materials are summarized in Table 3. It can be observed that the CO2 capture and O2 carrying capacity of the prepared composites decreases in the multi-cycle runs. This could be due to the substantial changes in distribution of Cu/CuO on the surface of CaO within the composites [14]. The loss in CO2 capture and O2 carrying capacity is more pronounced for the Cu61-Ca24-CA15 composite. The high percentage of CuO in the composite may then deteriorate the reactivity of CaO (for CO2

capture) due to the sintering of Cu/CuO at high temperature [17]. In order to prevent the sintering of CaO and CuO, the Ca12Al14O33 inert support is introduced in the prepared composites [19-21]. The multi-cycle tests show that an increased amount of inert support reduces the sintering of CaO and hence maintains a stable CO2 capture capacity, a

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better stability is obtained for Cu57-Ca23-CA20 and Cu53-Ca22-CA25 composites. Among the prepared composites, Cu53-Ca22-CA25 containing 53 wt% CuO, 22 wt% CaO and 25 wt% of Ca12Al14O33 shows the best stability performance for the Ca-Cu looping process conditions tested.

Table 3. Summary of theoretical and experimental CO2 capture and O2 carrying capacity of the synthesized composites before and after TGA multi-cycles.

Composite Ratio

CuO/CaO

CO2 capture capacity (g-CO2/100g composite)

O2 carrying capacity (g-O2/100g composite) Theoretical Experimental Theoretical Experimental

1^st cyc last cyc 1^st cyc last cyc

Cu61-Ca23-CA15 (aggl.) 2 19.0 19.0 10.0 12.2 9.5 4.2

Cu57-Ca23-CA20 (pellet) 2 18.0 17.9 15.8 11.4 10.2 5.4

Cu57-Ca23-CA20 (aggl.) 2 18.0 18.0 15.5 11.4 9.9 5.3

Cu53-Ca22-CA25 (pellet) 2 17.0 16.2 15.5 10.6 9.6 7.5

Cu53-Ca22-CA25 (aggl.) 2 17.0 16.5 15.1 10.6 9.8 7.2

To assess the decrease in CO2 capture and O2 carrying capacity of the composite materials in multi-cycle tests, a cross sectional study with SEM/EDS mapping has been performed for material (Cu53-Ca22-CA25) before and after test.

Fig. 5. SEM/EDS mapping of cross section images of the composite Cu53-Ca22-CA25; (a) fresh (before cycling) and (b) used (after cycling) EDS mappings show the distribution of different elements in the composite before and after cycling, respectively.

SEM micrographs and their corresponding EDS mapping (Fig. 5a) of the fresh sample show that the composite particles are well separated and Cu particles (bright part) are almost homogeneously distributed all over the composites. On the other hand, SEM/EDS mapping of the same composite after long term TGA cycling (50 cycles) shows that constituent particles are connected to each other due to sintering (Fig. 5b).

Fig. 5a and 5b clearly show that Cu/CuO has migrated towards the surface of the composite and agglomerated to form a thin layer after multi-cycling. This thin layer wraps over the CaO grains, and hinders partially the diffusion of CO2. The development of this Cu/CuO layer seems to stabilize with successive cycles and results in a stable CO2

capture and O2 carrying capacity when the CuO content is reduced and the sintering is moderate.

Mechanical strength is related to the particle’s composition and to the preparation method. Mechanical properties of the composite particles have been evaluated by measuring their crushing strength. Crushing strength of the composite particles was measured on 100 particles with a particle size between 0.8 and 1 mm. The median crushing strength of the prepared composites is summarized in Table 4. As expected, the composite Cu53-Ca22-CA25

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containing 25 wt% Ca12Al14O33 inert support shows the best crushing strength (11 N). The crushing strength has also been measured for a calcined limestone and a commercial reforming catalyst as reference materials.

Table 4. Crushing strength measurements on three different CuO-CaO-Ca12Al14O33 composites and reference materials

Material Preparation method Median crushing

strength, (N) Cu61-Ca24-CA15 Aggl. with 15 wt% PEG, calcined 1273K, 1h 6.1 Cu57-Ca23-CA20 Aggl. with 15 wt% PEG, calcined 1273K, 1h 6.0 Cu53-Ca22-CA25

Rheinkalk limestone Commercial reforming catalyst

Aggl. with 15 wt% PEG, calcined 1273K, 1h Calcined 1073K, 2h

11.0 9.0 27.0

4. Conclusions

A hydrothermal preparation method was used to synthesize from low cost precursors CaO-CuO composites supported on a mayenite (Ca12Al14O33) inert. The best results have been obtained for the composite material containing 53 wt% CuO, 22 wt% CaO and 25 wt% Ca12Al14O33, and show a stable CO2 capture and O2 carrying capacity of ~15 g-CO2/100g and ~7 g-O2/100g respectively in a simulated Ca-Cu looping multi-cycle. The observed migration and agglomeration of Cu/CuO on the surface of the particles reduce the CO2 capture and O2 carrying capacity. However, this effect can be minimized by reducing the CuO loading. A crushing strength of 11N has been obtained for this material, showing satisfactory mechanical stability compared to reference materials used in the Ca- Cu process. This new bi-functional CaO-CuO composite material may reduce the material costs of the Ca-Cu looping process due to the reduction of the overall inert material content.

Acknowledgements

The presented work is funded within the ASCENT project as part of the European Union’s Seventh Framework Program (FP7/2007-2013) under grant agreement nº 608512. Note: “The present publication reflects only the authors’ views and the European Union is not liable for any use that may be made of the information contained therein”.

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